That's a silly rubric. Loosely organized groups have cells with leaders, not a single one.
That specific Antifa chapter's cell leaders can be found in the link above.
The KKK didn't have a single leader either after WWII, it was distributed chapters. So does that mean they weren't a terrorist group anymore?
And the Juggalo fans didn't coordinate to go riot / cause violence, not the same as either of those.
The leaders of the Klan are well known: https://en.wikipedia.org/wiki/Leaders_of_the_Ku_Klux_Klan
It’s the rubric the government is using here: Trump designated the mythical centralized organization as a terrorist threat.
How is membership or leadership determined? How is a cell determined in this case?
In the case of the Klan (which you conveniently bracket to only after WW2) it, in fact was an organization with a single "Imperial Wizard" at the helm before being broken up in the 20's. The "distributed chapters" (themselves organizations with membership roles and defined leadership) were only "distributed" after the national organization was dissolved to bow out of a legal battle with the IRS.
There absolutely exist fans of ICP who have coordinated to cause violence, crime, and mayhem. (Though I don't believe that people should be prosecuted or persecuted for enjoying a goofy rap rock band)